1. Field of the Invention
The present invention relates to self-adhesive carbonaceous grains and high density carbon artifacts derived therefrom. More specifically, the present invention relates to self-adhesive carbonaceous grains which are suitable as a starting material for the production of high density carbon artifacts and a binding substance for high performance carbon-carbon composites. The present invention and relates also to high density carbon artifacts produced from such carbonaceous grains.
2. Description of Related Art
High density carbon artifacts have been widely applied in various fields because of their excellent properties. They are now recognized to be indispensable for the most advanced technologies.
So far, many studies of the production of high density carbon artifacts have been made. The processes for preparing carbon artifacts are typically classified into two categories.
Procedures in the first category use filler coke and binder pitch as starting materials. Natural graphite and carbon black can be also used as filler. Pulverized coke may be used as a filler when blended with coal-tar or pitch as a binder. The process consists of molding of the mixture, repeated carbonization (baking) followed by impregnations for densification, and graphitization into the final product. In this method, there are several shortcomings. Since the carbonization yield of the binder is very low, cumbersome secondary treatments involving multiple impregnation and carbonization of the binder for densification is necessary. Further, an extremely slow heating of 2.degree.-10.degree. C./h is required during the carbonization process since the rapid evaporation of volatile matters causes expansion of or void formation in the product. Therefore 3-4 weeks are required to complete the carbonization process.
The carbonized product may be graphitized by further heating to a temperature in the range of 2500.degree.-3000.degree. C., depending upon the use of the final product and upon the specific performance required of it. Generally this process also requires a long period of 2-3 weeks to complete. Therefore, a considerably longer process time is necessary to produce high density carbon artifacts through the conventional, complicated method of using a filler and a binder.
The second process consists of preparation of self-adhesive carbonaceous grains, molding them without any additional binding substances, carbonization and graphitization. The absence of the impregnation step is a characteristic of this process. Mesocarbon microbeads have also been developed as a starting material for binderless molding, as described in GBP 1,324,124 and U.S. Pat. No. 3,956,236.
Mesocarbon microbeads are prepared through a heat treatment at 350.degree.-500.degree. C. of coat-tar or petroleum based pitch, solvent-extraction, filtration from the matrix, drying, and classification. High density isotropic carbon artifacts can be obtained by molding such mesocarbon microbeads under pressure and subsequently calcining them. However, there are a few problems to be solved in connection with this method.
This process entails the use of a large quantity of solvent duing the extraction and filtration steps. In addition, it is very difficult to completely remove remaining solvent from mesocarbon microbeads obtained in this way. This means that cracking or expansion of the product is apt to result during the carbonization. Furthermore, in addition to the problem of low extraction yield in this process, it is also difficult to properly control the properties of the mesocarbon microbeads.
As described above, both conventional processes for producing high density carbon artifacts are very disadvantageous from the viewpoint of productivity and economy. These fabrication processes for high density carbon artifacts are extremely complicated and the final products prepared by such conventional methods are very expensive. As a result, their broad application is greatly restricted at this stage.